Glasses that are capable of producing changes in color are sometimes referred to as “polychromatic” glasses. Glasses that are capable of producing refractive index modulation upon illumination by light, followed by thermal treatment, are sometimes referred to as “photorefractive” or “photo-thermal-refractive” (PTR) glasses. Such glasses are well known, and may be referred to generally herein as “holographic” glasses. Examples of such holographic glasses have been disclosed in, for example, U.S. Pat. No. 4,017,318 (“Pierson”), U.S. Pat. No. 4,514,053 (“Borrelli”), U.S. Pat. No. 6,586,141 (“Efimov”), and Soviet patent SU 1,780,429 (“Borgman”). The entire disclosures of each of the foregoing patents are incorporated herein by reference.
A characteristic of a holographic glass is the optical performance, in a very general sense, of the glass at a certain frequency or wavelength of light, or over a range of frequencies or wavelengths of light, before and after a hologram has been recorded in the glass. The performance of such a holographic glass may be measured by a so-called “figure of merit” (FOM). For better optical performance, it may be desirable to increase the FOM of holographic glasses.